The present invention relates to kinematic couplings, particularly to a kinematic coupling for precision instruments, and more particularly to a highly damped kinematic coupling for precision instruments which utilizes three ball-cone constraints each combined with a released flexural degree of freedom and which include constrained-layer damping.
Load-sensitive apparatus, such as the projection optics system for extreme ultraviolet lithography (EUVL) requires precise alignment of the reflective optics within the structural housing. It is critical that the housing be isolated from variable loads that could change the optical alignment. Such loads could arise from being mounted to other structures in an overconstrained manner. Each constraint of the six rigid-body degrees of freedom may be achieved approximately with a kinematic coupling. Kinematic couplings have long been used for the purpose of repeatable location and minimal influence to the supported object. Friction present in the contacting surfaces of a typical kinematic coupling, for example, a three-vee coupling, introduces small but uncertain forces that distort the object being supported. In addition, the typical kinematic coupling provides very little damping at low excitation levels.
The present invention provides the same kinematics as the three-vee coupling by using three ball-cone constraints each combined with a released flexural degree of freedom. This three ball-cone arrangement enables higher load capacity and stiffness, but can also significantly reduce the friction level in proportion to the ball radius divided by the distance between the ball and the hinge axis. The coupling of this invention also utilizes blade flexures which reduce somewhat the stiffness of the coupling and provides an ideal location to apply constrained-layer damping, which includes a viscoelastic layer and a constraining layer on opposite sides of the two blade flexures of the coupling. Constrained-layer damping has been previously utilized as a damping treatment across flexures of kinematic couplings, see "Highly-Damped Exactly-Constrained Mounting Of An X-Ray Telescope," P. S. Wilke et al., SPIE, Vol. 2445, Passive Damping, March 1995. It is reasonable to achieve one to two orders of magnitude reduction in the vibrational amplitude with the coupling arrangement of the present invention.